Conventional digital copiers include those equipped with a document feeder such as an automatic document feeder (referred to as an ADF below) capable of dealing with double-sided copying.
When a document that has been supplied to the ADF is copied in a digital copier having an ADF of this kind, a reading unit provided below the ADF is moved by a motor and stopped at a set position, namely a point substantially directly beneath a platen roller within the ADF, after which light is emitted from a lamp unit and the document transported between the platen roller and a glass plate to thereby scan the document with the light so that light reflected from the document may be detected by the reading unit. A plurality of images are obtained by executing processing in a printer section based upon data obtained by such detection.
When the reading unit provided below the ADF is moved to the set position by the motor in the conventional arrangement described above, the motor is stopped at such time that a predetermined number of pulses have been counted starting from the moment at which a position sensor senses the reading unit, and the stopping position is adopted as being the reading position. A problem which arises is that the precision of the reading apparatus declines (on the order of about ±2 mm), causing a decline in image quality at reading of the document, owing to differences in the mounting position of the ADF or a variance in the reading position of the ADF relative to the position of the reading unit.
The specification of Japanese Patent No. 2993810 proposes means for solving this problem, namely means for moving a reader in a sub-scan direction and, while the reader is being moved, adopting as the reading position a position that maximizes a value obtained by reading light entrant upon being reflected from the platen roller.
However, it has been found that as a result of reading reflected light from the platen roller using the reader, there are instances where a plurality of positions that give maximum values exist owing to the diameter and surface conditions of the platen.
Further, the actual path traversed by a document varies depending upon document thickness. In addition, with regard to documents having a glossy surface, there are instances where the value obtained by reading reflected light from the platen roller while the reader is being moved in the sub-scan direction is not necessarily the optimum reading position.